For person authentication, conventionally, password authentication and identification (ID) card authentication have been widely used. However, such authentication using non-biometric data has a high risk of fraudulent use. From this viewpoint, as more reliable person authentication, authentication using biometric data (biometric authentication) such as fingerprint authentication has attracted attention and tends to be used.
However, in the biometric authentication, a problem of false acceptance due to a fake imitating a biologic object has been pointed out. In the fingerprint authentication, for example, it is pointed out that many fingerprint sensors determine a gummy finger, a fake finger made of gelatin, as a live finger.
To prevent such abuse using fake fingerprints, a technology is known to generate an oscillation frequency corresponding to an electrostatic capacity of an object under test, e.g. a live finger, to detect whether the object under test is a biologic object or not by comparing the oscillation frequency to a reference signal, as disclosed in Japanese Laid-open Patent Publication No. 10-165382, for example.
Further, another technology is known to apply two square-wave input voltages having different frequencies to an object under test and calculating an impedance of the object under test on the basis of the output voltage to determine that the object under test is a live finger in accordance with whether the impedance is within an impedance range of live fingers, as disclosed in Japanese Laid-open Patent Publication No. 2005-143804, for example.
Further, to deal with a case where a live finger is not determined as a live finger because a measured value of the live finger is lower than a biologic object determination reference value, another technology is known to determine whether a live finger or a fake finger by changing a biologic object determination reference level depending on the state of each biologic object and the biologic object detection function, as disclosed in Japanese Laid-open Patent Publication No. 10-302047, for example.
In determination whether a live finger or a fake finger, technologies based on an electrostatic capacity of a live finger (e.g. Japanese Laid-open Patent Publication No. 10-165382) or technologies based on an impedance of a live finger (e.g. Japanese Laid-open Patent Publication No. 2005-143804) have inconvenience that a biologic object having a characteristic similar to a fake finger may be erroneously determined as a fake finger.
On the other hand, setting a threshold for determining whether a live finger or a fake finger to a small value under a condition where it is highly possible that a live finger is determined as a fake finger (e.g. Japanese Laid-open Patent Publication No. 10-302047) make a live finger not to be determined as a fake finger but make a fake finger to be easily authenticated, which causes a problem in security.
Some biologic objects have similar characteristics as the fakes. In such a case, forgery similarities are high. Accordingly, impersonation with a fake for a biologic object easy to forge may be easy when forgery determination thresholds are uniformly lowered.
As an example of a fake, there is a gummy finger made of gelatin. To determine whether a gummy finger or a biologic object, there is a method based on impedance. In the method based on impedance, a range of impedance of live fingers (or fake fingers) is recorded in advance. Whether a live finger or a fake finger is determined in accordance with whether input impedance of the object under test is within the range. Impedance of a biologic object differs depending on a state of the finger skin such as dry, wet, or the like. A gummy finger has different impedance depending on production methods or the like. Accordingly, some live fingers (wet fingers, etc.) have impedance characteristics similar to some of gummy fingers. For live fingers that have impedance characteristics similar to such gummy fingers, if the impedance range is set such that the live fingers are not erroneously determined as fake fingers (gummy fingers), it is difficult to prevent false acceptance by the gummy fingers. Further, there is a problem that live fingers having similar impedance characteristics as gummy fingers may be erroneously determined as fake fingers.